The invention relates to a belt tensioner for a vehicle occupant restraint system comprising a tube containing gas in the trigger case and an igniter unit arranged in the tube which is inserted in an open end face of the tube and includes a base and a cap fastened to the base for accommodating a propelling charge, the cap being arranged at least partly inside the tube.
Such belt tensioners are known. In a case of restraint the igniter unit is activated and generates a pressure gas which is forwarded in the tube and drives a turbine coupled to a shaft of a belt retractor, for example, so as to draw in a belt slack. In the case of those belt tensioners the turbine and, resp., the drive wheel is either directly pressurized by the pressure gas or one or more mass bodies provided in the tube are moved by the pressure gas in the direction of the driving wheel and load the latter. Such belt tensioners are known, for example, from DE 10 2010 051 420 A1 or U.S. 2001/0035472 A1. The igniter unit usually is a prefabricated sub-assembly which is fixed in the tube by means of a dovetail joint or screw joint.